Wireless communication is used in a large variety of technical fields. Typical examples are mobile phone, wireless LAN, walkie-talkies, radio systems and point-to-point radio systems.
The communication radius covered by a respective communication system basically depends on the technique used. Whereas GSM and UMTS are adapted for a communication radius up to about 10 km, wireless LAN frequently is restricted to about 100 m and the Bluetooth system usually is limited to about 20 m. The major influences on the communication range of a communication technique are the transmission frequency and output power used. Whereas only little absorption of electromagnetic waves in the atmosphere occurs at the transmission frequencies used for GSM/UMTS a significant absorption occurs in the 60 GHz range. Thus, the 60 GHz range suits best for low and middle communication radiuses.
Furthermore, the kind of antenna used for a respective wireless communication technique varies depend on a respective field of application.
If large numbers of receivers have to be reached or the location of the receiver is unknown or varies frequently, wide-beam antennas are used.
On the other hand, if only one or at least a very limited number of receivers have to be reached and the respective receiver(s) is/are stationary or at least quasi-stationary narrow-beam antennas can be used.
The utilisation of wide beam antennas in high data rate systems (e.g. over 1 Gbps) is problematic because of the multi-path fading effect. This multi-path fading effect is caused by differences of travelling time between radio wave paths of the same transmission, as experienced at the receiving station.
The multi-path fading effect is shown in FIG. 12.
As shown in FIG. 12, if wide beam antennas 121, 123, with e.g. a half power beam width (HPBW) of 100° are used for both a first station 120 at a sending side and a second station 122 at a receiving side and a line-of-sight 12f (LOS) communication path is blocked by an obstacle 124, there exist a lot of reflection paths 12a, 12b, 12c, 12d and 12e between the first station 120 and the second station 122 due to a plurality of reflecting surfaces 125, 126, 127, 128 and 129. The channel delay spread might be over tens of symbol periods when the data transmission rate is high (e.g. over 1 Gbps), which leads to severe inter-symbol interference (ISI) due to deep frequency selective fading.
As it is obvious from FIG. 12, the multi-path fading effect most likely occurs in city-centres or in indoor environments where a plurality of reflecting surfaces 125, 126, 127, 128 and 129 (e.g. walls) are present.
Two conventional solutions exist for such kind of a no-line-of-sight (NLOS) user scenario:
One adopts channel equaliser including a linear, a decision feedback or a maximum likelihood sequence estimation (MLSE) equaliser. When the channel delay spread is much longer than the symbol duration, the equaliser becomes complex and needs a lot of processing power.
Another solution is the orthogonal frequency division multiplexing (OFDM) technique, which is already adopted in wireless LAN systems. However, due to its inherent linear modulation and high peak to average ratio problems, the power consumption of a power amplifier (PA) used with the OFDM technique is very high.
Thus, both solutions need high-speed and complex signal processing circuits.
In order to reduce the channel distortion, the adoption of a wide beam antenna 123 at one side and a sharp half power beam width (HPBW) steering antenna at another side for a no-line-of-sight (NLOS) user scenario is known as it is shown in FIG. 13.
The wide beam antenna 121 of the first station 120 of FIG. 12 is replaced by a sharp beam antenna 131. Said sharp beam antenna 131 is steered to the optimum position (with feasible steering resolution), which could match to the strong reflection path 12b and 12c caused by the reflecting surfaces 127 and 128. Due to the usage of said sharp beam antenna 131, the reflection signals 12a, 12d and 12e shown in FIG. 13 are not generated and thus can not reach the wide beam antenna 123 of the second station 122. Therefore, the channel delay spread is shortened.
Furthermore, another system concept is to use a pair of sharp beam steering antennas 131 and 143 on both the transmitting first station 120 and the receiving second station 122 side, which can be seen in FIG. 14.
Both sharp beam steering antennas 131 and 143 can be steered to an optimum position, where strong reflection signals 12c caused by a reflecting surface 128 can be transmitted and received by both sharp beam steering antennas 131 and 143 of the first and second station 120, 122. As a result, the reflection signals 12a, 12b, 12d and 12e shown in FIG. 14 are not generated and thus cannot reach the second station 122. Thus, the channel delay spread can be further shortened. In addition, considering the additional antenna gains obtained by both sharp beam antennas 131 and 143, a strong received signal 12c with a relatively small frequency selective fading channel can be obtained.
The usage of narrow beam antennas as it is described with respect to FIGS. 13 and 14 has the disadvantage that due to the limited beam width emitted by a narrow beam antenna tracking of both antennas is very difficult in no line of sight scenarios. Another problem is that a fast steering of the narrow beam antennas is necessary after a loss of the direct or indirect communication path has occurred in order to keep the data rate of the respective wireless communication system.
In summary the state of the art suffers from the following disadvantages:
Communication systems utilising wide beam antennas have to cope with the multi-path fading effect. This effect is even amplified when using high data rates. To overcome the multi-path fading effect, complex equalisers or complex modulation scheme e.g., OFDM are required.
Communication systems using narrow beam antennas have fewer problems concerning the multi-path fading effect. With communication using systems narrow beam antennas it is very difficult to support communication under non-line-of-sight conditions. Moreover, in a communication system with narrow beam antennas on both sides, the replacement of a broken link is very time consuming since a new suitable communication path has to be searched. This results in a significant decrease of the data transmission rate.